1. Field of the Invention
This invention relates to circuit breakers and, more particularly, it pertains to a high capacity expulsion fuse.
2. Description of the Prior Art
Circuit interrupters, such as boric acid fuses for high voltage operation above about 15 kV, comprise a tube of gas evolving, arc-extinguishing material, such as boric acid, through which a movable contact rod extends. One end of the rod is retained against a spring by a fusible wire. In order to be a full range interrupting device, that is, one which can interrupt the full range of fault currents from minimum melting to maximum interrupting rating, certain minimum clearance parameters must be maintained between the rod and the surrounding boric acid tube. The minimal clearance is required during the interruption of low currents on the order of from one to 800 amperes. The minimal clearance is needed so that an adequate pressure is generated during interruption of these relatively low currents. If the clearance exceeds the required parameters, the required dielectric strength of the interrupting medium, during arcing, is not realized nor is the ionic disassociation which results from a high pressure gas blast effective.
The minimal clearances, however, have caused problems when water seeps into the fuse. Ordinary current carrying operation of a fuse at elevated temperatures can cause a transfer of water vapor and migration of dissolved boric acid which affect the required clearances. The migration of boric acid has been found to cause an actual physical interference between the contact rod and the boric acid inner diameter, resulting in the contact rod becoming "frozen" in place, whereby the fuse rod is unable to retract during a fault interruption operation. Without physical withdrawal of the contact rod, the fuse fails to interrupt and destructs.